As well known, Content On Demand is a bidirectional service which enables service subscribers to request and receive desired content (video, music, file, image, and website information) from a service provider at desired times, and Video On Demand (VOD), the provision of which is currently increasing, may be referred to as a representative of COD.
For reference, VOD systems generally provide all the functions of general video devices, other than the fact that video information is ordered over a network, that is, functions of playing, stopping, pausing, fast forwarding, fast rewinding, and starting to play at an arbitrary portions. According to the type of provision, services provided by such VOD systems are classified into a Real COD (RCOD) service, in which a pre-determined charge is paid for each program, a Subscription COD (SCOD) service, in which a fixed charge is paid for one month while program packages are viewed without limitation as to the number of views, and a Free-On-Demand (FOD) service, which is supplied as marketing means for free.
A COD system, as shown in FIG. 1, includes a COD server 10, a COD client 20, and a network 30 for connecting them to each other.
The COD server 10 includes a high-capacity content database, and performs functions of searching for content requested by the COD client 20 and transmitting the corresponding content to the COD client 20.
The COD client 20 restores and plays the content transmitted from the COD server 10, and transmits the request of a user to the COD server 10.
The network 30 is means for connecting the COD server 10 and the COD client 20 such that communication therebetween is possible, and may include a well-known broadcasting network or information network.
Referring to FIG. 2, a method of operating a prior art Content-On-Demand system will be described stepwise below.
1. The step of receiving a request for content from a COD client 20.
2. The step of determining whether the requested content exists in the COD server 10, and, if the requested content does not exist, informing the corresponding COD client 20 that the requested content does not exist.
3. The step of determining whether the requested content exists in the COD server 10, and, if the requested content exists, providing access information for one-to-one transmission (unicast) to the corresponding COD client 20, so that a session is set up between the COD server 10 and the corresponding COD client 20, transmitting the requested content from the COD server 10 to the corresponding COD client 20 in a one-to-one transmission manner, and, when the transmission is completed, terminating the session between the COD server 10 and the corresponding COD client 20.
Referring to FIG. 3, the method of operating a Content-On-Demand system according to the related art will be described in further detail below.
With reference to the drawing, the COD server 10 is configured to include a content management unit 11 configured to have a content database and provide content to a content transmission unit 12; the content transmission unit 12 configured to transmit content, provided from the content management unit 11, to one or more COD clients 20a, 20b, 20c, and 20d over a network 30; and a control unit 13 configured to set up/terminate sessions between the COD server 10 and the respective COD clients 20a, 20b, 20c, and 20d, and control the operation of the content management unit 11 and the content transmission unit 12.
First, when a content request signal, provided from a first COD client 20a, is input to the COD server 10 over the network 30, the control unit 13 determines whether the requested content exists in the content database of the content management unit 11. If the requested content is found not to exist, the control unit 13 transmits a message informing the first COD client 20a that the requested content does not exist. If the requested content is found to exist, the control unit 13 transmits access information for one-to-one transmission, which allows access to the transmission-capable region of the content transmission unit 12, to the first COD client 20a, so that a session is set up between the COD server 10 and the first COD client 20a. 
If the session is set up between the COD server 10 and the first COD client 20a, the control unit 13 controls the operation of the content management unit 11 and content transmission unit 12 and transmits the requested content to the first COD client 20a using the one-to-one transmission-capable region of the content transmission unit 12, for which the session has been set up.
When the transmission of the requested content from the COD server 10 to the first COD client 20a is completed, the control unit 13 terminates the session between the COD server 10 and the corresponding COD client 20 and completes a task for providing the content to the first COD client 20a. 
Thereafter, if content is sequentially requested by a second COD client 20b and a third COD client 20c, the content is provided to the corresponding COD clients 20b and 20c through the above-described task in a one-to-one transmission manner.
However, when content is requested by one or more COD clients 20 (20a, 20b, 20c, and 20d), the prior art system uses a method of setting up sessions between the COD server 10 and the respective COD clients 20 (20a, 20b, 20c, and 20d) such that one-to-one transmission is possible between the COD server 10 and the respective COD clients 20 (20a, 20b, 20c, and 20d) regardless of the type of content requested by the respective COD clients 20 (20a, 20b, 20c, and 20d). Therefore, even though the COD clients 20a, 20b, 20c, and 20d, which are different from each other, request the same content, the regions of the content transmission unit 12 are separately allocated to the respective COD clients 20a, 20b, 20c, and 20d. Accordingly, if specific content (for example, a popular video or the latest video) is overwhelmingly requested for a short time, the capacity of the COD server 10 is exceeded, so that a problem occurs in that a server goes down.
In order to solve the above-described problem, in the prior art, the number of separate servers dedicated to the provision of popular content which is overwhelmingly requested for a short time must be increased, or the capacity of the region of the COD server 10 must be expanded and the bandwidth of a network must be extended so as to prepare the temporal access congestion. However, in these cases, a problem occurs in that investment must be continuously made in order to construct a stable COD system in response to the increase in the number of COD service users.